1. Field of the Invention
This invention relates to an improvement of a magnetic recording medium provided with at least one layer of obliquely deposited magnetic crystals wherein ferromagnetic columnar grains are formed on a substrate at an oblique angle with respect to the substrate by causing a stream of vapor obtained by heating and evaporating a ferromagnetic material to obliquely impinge upon the substrate in a vacuum.
2. Description of the Prior Art
There is nowadays a need for a magnetic recording medium able to meet the strong demand for high density recording. Particularly, various efforts are being made to develop a thin metal film type magnetic recording medium suitable as a high density magnetic recording medium. Thin metal film is formed by various methods such as vacuum deposition, ion plating, sputtering and wet plating. However, as a method capable of consistently producing a high coercive force medium suitable for high density recording, the oblique incidence vacuum deposition method as disclosed in Japanese Patent Publication No. 41(1966)-19389 is especially good.
In the oblique incidence vacuum deposition method, a stream of vapor obtained by heating and evaporating a ferromagnetic material such as Co, or Co--Ni is caused to impinge upon a non-magnetic substrate made of a high-molecular material or the like in a vacuum generally at an angle of 45.degree. or more with respect to the line normal to the substrate, thereby forming ferromagnetic columnar grains on the substrate at an oblique angle with respect to the substrate. In order to obtain a high coercive force of about 1,000 Oe by use of a Co material or a Co--Ni material as the deposition material, it is generally necessary that the minimum angle of incidence (.theta.min) be about 60.degree. or more.
However, when oblique incidence vacuum deposition is conducted at a minimum angle of incidence (.theta.min) of 60.degree. or more, the portion of the vapor stream interrupted by a mask is increased, and the amount of the vapor stream deposited on the substrate is decreased markedly, resulting in a marked decrease in the deposition efficiency and deposition speed.
To eliminate the drawbacks of the aforesaid oblique incidence vacuum deposition method, it has been proposed to conduct oblique incidence vacuum deposition in an oxygen atmosphere. In the method of conducting oblique incidence vacuum deposition in an oxygen atmosphere, it is possible to obtain a deposited magnetic layer exhibiting a high coercive force and, consequently, decrease the minimum angle of incidence (.theta.min) in the deposition process. Namely, this method is suitable for increasing the coercive force of the obtained magnetic layer and decreasing the angle of incidence of the vapor stream of the deposition material. However, the obliquely deposited Co or Co--Ni film formed by the oblique incidence vacuum deposition method or the method of oblique incidence vacuum deposition in an oxygen atmosphere exhibits insufficient resistance to rusting, which constitutes an obstacle to putting the method to practical use.
In order to improve the resistance of the magnetic layer to rusting, it has been proposed to add Cr to the deposition material. However, to obtain a desired level of rust resistance, it is necessary to add Cr in a large amount, for example, in an amount of 10 atom % or more. As a result, the magnetic flux density and the electromagnetic transducing characteristics of the obtained magnetic layer become very low.